1. Field of the Invention
The present invention relates to stimulus-indicating techniques. More particularly, the present invention relates to devices, methods and kits for indicating when a product has been exposed to a given stimulus for a certain period of time.
2. Background of the Invention
It has long been recognized that the useful life of a perishable product is a function of its cumulative exposure to a harmful stimulus over a period of time. The types of stimuli that advance the degradation of stimulus-sensitive products over time include not only temperature, but also light, pH, humidity, electromagnetic radiation and radiation, to name a few. For example, the useful life of a temperature sensitive product is a function of its cumulative time-temperature exposure a combination of the temperature to which a product is exposed and the duration of the exposure.
For stimulus sensitive products, degradation generally occurs more rapidly at higher magnitudes of the stimulus (e.g., higher temperatures) than at lower magnitudes of the stimulus (e.g., lower temperatures). For example, often a temperature-perishable product will have a longer useful life if it is exposed to lower temperatures than if it is exposed to higher temperatures. However, the converse is also true, in that certain stimulus sensitive products will degrade more rapidly at a lower magnitude of stimulus (especially freezing temperatures) than at a higher magnitude of stimulus. For example, a malaria vaccine will degrade faster below 2° C. than if stored at 3° C.-7° C. Thus the rate of degradation is often stimulus and product specific.
Stimulus perishable products susceptible to degradation due to cumulative time-stimulus exposure include, but are not limited to, food, food additives, chemicals, biological materials, drugs, cosmetics, photographic supplies and vaccines, to name a few. Many manufacturers mark their products with printed expiration dates in an attempt to provide an indication of when the useful life of a perishable product lapses. However, these dates are only estimates and may be unreliable because they are based on assumptions about the stimulus history of the product that may not be true with respect to the particular unit or product within the package on which the expiration date appears.
Specifically, manufacturers compute expiration dates by assuming that the product will be kept within a specified stimulus range during the product's useful life. However, although the manufacturer may have certain control over the environmental conditions of the product while the product is in its possession, it cannot always predict or control the stimulus exposure of a product through each step of the supply chain that delivers the product from its possession to the consumer. If the product is actually exposed to stimuli greater in magnitude than those on which the expiration date is based, the perishable product may degrade or spoil well before the printed expiration date. When this happens, the expiration date may mislead the consumer into believing that the perishable product is still usable when, in fact, its useful life has lapsed. Such false belief of the usefulness/effectiveness of such a product could lead to medical harm, injury or death for a consumer.
Despite its limitations, marking a stimulus sensitive product with an expiration date is useful, nonetheless, because often a visual inspection of the perishable product does not warn a potential user that the exposure of the product to a harmful stimulus has caused it to degrade or spoil. Although there are certain time-stimulus indicators currently available, most are focused on temperature, and not on the other types of stimuli. Additionally, these time-temperature indicators require active agents to be kept separate from one another until the indicator is attached to the product it is monitoring. If the active ingredients are not kept separate they will begin to interact prematurely, thereby giving a premature indication of spoilage. As a result, these types of indicators require manual activation. However, manual activation is not always feasible, cost-effective or efficient when the indicator is being used with a product that is mass-produced in high-volume.
Thus, there is a need for a stimulus magnitude indicator that indicates when a stimulus sensitive product has been exposed to a deleterious stimulus extreme for a pre-determined amount of time. There is also a need for a time-stimulus indicator that does not contain active agents that will begin to interact prior to being attached to the product being monitored, thereby resulting in false indications of product expiration. Furthermore, there is a need for a stimulus indicator that does not require manual activation, and a stimulus indicator that can indicate exposure to two or more temperature magnitudes in a single device.